The invention relates to a water-soluble, water-softening builder in the form of a tablet.
Water-softening builders in tablet form are widely known. Tablets have a variety of advantages over powder products: they are easier to dose and handle and can be added more easily to the laundry, they are more compact and permit cost-effective storage.
Water softening tablets are sold commercially (Calgon 2-phase tabs) and such tablets are described in EP-A1-0 628 627 (Benckiser), CH-577 937 (Lonza), WO-A1-95/21908 (Henkel) and EP-A2-0 622 449 (Hxc3xcls).
Water-softening tablets are usually prepared by compressing or compacting water-softening powders. It is, however, difficult to find a balance between the required hardness and abrasion resistance of the tablets and their property of disintegrating and/or dissolving quickly in the wash liquor. Tablets which have been prepared using only a slight compacting pressure tend to crumble and to break up during handling and packaging; tablets which have undergone greater compaction are more durable, but do not disintegrate or disperse in the wash liquor as well either.
A variety of solutions have already been proposed, particularly for detergent tablets, for improving their physical properties (strength, abrasion resistance and dispersibility).
GB 983243 and GB 989683 (Colgate-Palmolive) disclose detergent tablets having improved dissolution properties, which have been prepared by compaction of spray-dried detergent powders and spraying with water or aqueous sodium silicate solution to reduce the proportion of fine particles. The whole tablet is coated with a film-forming polymer to improve the abrasion resistance. The use of polyacrylic acid is not disclosed.
EP-A-0 466 484 (Unilever) discloses detergent tablets comprising compacted particles having a narrow size distribution and uniform and regular particle size. This results in the advantage of an attractive appearance and better disintegration in the wash liquor.
EP-A2-0 522 766 discloses detergent tablets comprising compacted particulate detergent compositions in which at least some of the particles have been coated with a material which acts as a binder, but which also promotes disintegration in the wash liquor. Although polymers are claimed as suitable disintegrant additives, polyacrylic acid is not disclosed.
EP-A2-0 711 828 (Unilever) discloses a process for the preparation of detergent tablets by compaction of particulate detergent compositions in which a binder is distributed which has a melting point in the range of 35-90xc2x0 C., where compaction to give tablets takes place at a temperature above 28xc2x0 C. but below the melting temperature of the binder. Polyacrylates are mentioned as possible binders, but polyacrylic acid is not disclosed.
Polymers and copolymers of acrylic acid are known constituents of detergents and water softeners. It has also already been proposed to add alkali metal salts of such polymers or copolymers to tablet formulations.
WO-A1-92/18604 (Henkel) discloses a process for the preparation of detergent tablets for dishwashers, where compositions comprising alkali metal polyacrylate are agglomerated and dried in a fluidized bed.
WO-A1-93/00419 (Henkel) discloses a process for the preparation of detergent tablets for dishwashers, where a prehydrated mixture of alkali metal polyacrylate and sodium carbonate is prepared, which is then mixed with the other components of the tablet.
WO-A1-95/21908 (Henkel) discloses tablets which comprise builders, including layer silicates and which have a reduced water content and are reportedly notable for good solubility in water. Salts of polyacrylates are mentioned here as disintegrants.
EP-A1-628 627 (Benckiser) discloses a water-soluble, water-softening builder in the form of a tablet consisting of
A) 60-98% by weight of a combination of
a) citrate and/or citric acid and
b) a polymer which acts as a water softener in an a:b weight ratio of from 70:30 to 50:50, preferably from 65:35 to 55:45,
B) 0.5-6% by weight of polyethylene glycol
C) 0-38% by weight of other auxiliaries.
Although polyacrylic acid can be regarded as a polymer with water-softening properties, the use of polyacrylic acid is not expressly described in this publication.
The invention provides improved tablet properties (hardness, strength, abrasion resistance, solvency in the wash liquor) of water-soluble, water-softening builders in the form of a tablet. It has been found that this object can be achieved in tablets which comprise 40-98% by weight of a combination of a) an organic, multifunctional polycarboxylic acid or alkali metal salts thereof, and b) a polymer with water-softening properties, by using polyacrylic acid in acid form as the polymer with water-softening properties. In particular, the aim according to the invention is to use a polyacrylic acid having a molecular weight of from 1500 to 8000.
The invention thus provides a water-soluble, water-softening builder in the form of a tablet comprising 40-98% by weight of a combination of
a) one or more organic, multifunctional polycarboxylic acid(s) or alkali metal salts thereof, and
b) a polymer with water-softening properties in an a:b weight ratio of from 80:20 to 50:50, wherein the polymer with water-softening properties is a partially neutralized polyacrylic acid having a molecular weight of 1500-8000.
An organic, multifunctional polycarboxylic acid is taken to mean a polybasic carboxylic acid having additional functional groups, for example hydroxyl groups and/or amino groups. Suitable multifunctional polycarboxylic acids are gluconic acid, tartaric acid and citric acid. Preference is given to citric acid which, like the other acids, can be used in acid form or as an alkali metal salt.
The polyacrylic acid to be used according to the invention is taken to mean a homopolymer of acrylic acid which still contains free carboxyl groups. For the purposes of the invention, this term should also encompass a partially neutralized polyacrylic acid.
The term xe2x80x9cpartially neutralizedxe2x80x9d in this connection should, however, in any case exclude neutralization of more than 90% of the free carboxyl groups.
The polyacrylic acid to be used should have a molecular weight of from 1500 to 8000.
A suitable polyacrylic acid, which is commercially available, is the product Norasol LMW45D from Norsohaas. This is a partially neutralized polyacrylic acid having a molecular weight of 4500.
The builder tablet according to the invention can also comprise further customary additives, in particular it preferably comprises polyethylene glycol, preferably in an amount of 0.5-6% by weight. Suitable polyethylene glycols are those with a relatively high degree of ethoxylation, for example polyethylene glycols having a molecular weight of from 1000 to 6000.
The tablets according to the invention can further comprise customary tableting auxiliaries, such as, for example, microcrystalline cellulose and also sodium bicarbonate.
Hardness, solubility and abrasion resistance of the tablets are determined by the following methods of measurement:
The hardness of the tablets was determined using an Erichsen 486 hardness tester. The device measures the force which is necessary to break one tablet.
The solubility of a tablet is measured by immersing it inside a cage with 6 mm wide windows into a beaker containing 1 l of water at room temperature. The tablet is considered to be dissolved when all of the particles have passed through the windows of the cage.
The abrasion resistance of a tablet is measured by placing 10 tablets in a rectangular plastic container (170 mmxc3x97110 mmxc3x97110 mm) and rotating the container at a rate of 40 revolutions per minute for 1 minute. The amount of powder which is produced in the container is weighed and recorded as a percentage of the original weight of the tablet.